Before describing prior art, descriptions will be given of the CD-R, in particular its construction and how data is recorded in the CD-R.
Typically, the CD-R is a kind of CD-ROM designed to write data only once for saving the written data.
A construction of the CD-R is illustrated in FIG. 4 while a method of recording data on the CD-R is illustrated in FIG. 5. In FIG. 4, the CD-R is a round disc having an internal construction from the side to which a laser beam is directed a polycarbonate layer, an organic coloring layer, a gold plated layer, a protection layer and a printing label layer. In FIG. 5, when a laser beam is directed on the coloring layer from the polycarbonate layer, chemical reaction takes place in the coloring layer at the area to which the laser beam is directed to change the material to exhibit poor light reflecting property, thereby recording data pits. In other words, areas of the CD-R at which chemical reaction took place are considered pits which are essentially the same in meaning as pits in a CD-ROM. As a result, data is recorded as pits.
Because of the above mentioned construction and data storage principle, the CD-R has a characteristic of writing the data only once. The CD-R is therefore convenient for the user in that the written data cannot be revised. However, it is inconvenient in that the data cannot be easily erased and that companies using CD-Rs encounter difficulty in disposing of unnecessary CD-Rs. One conventional countermeasure is to scratch the surface to which a laser beam is directed for reading, thereby making it impossible to read out by a normal read-out method of the CD-R.
Unfortunately, scratching the laser directing surface of a CD-R not to be read out by a normal read-out method is not complete because the data is not basically destroyed and the data may be read out if the scratched surface is polished to remove the scratch. This suggests that the conventional erasing technique can not be used to erase the CD-Rs in which important proprietary data is stored.
On the other hand, the CD-R market is expanding rapidly and CD-Rs are forecasted to be widely used. This is the reason why it is desired to develop simple and more reliable erasing method and apparatus.